The structure of a conventional ordinary excavator is shown in FIG. 11. A work front 100 is made up of two members, i.e., a boom 101 and an arm 102. A bucket 103 for use in excavation work is provided at a fore end of the work front 100. The work front 100 is called a 2-articulation type work front because the bucket 103 serving as a main member to carry out the work is positioned by two rotatable structural elements, i.e., the boom 101 and the arm 102. An excavator provided with the work front 100 is called a 2-articulation type excavator.
Meanwhile, the so-called two-piece boom type excavator has been employed recently. One example of the two-piece boom type excavator is shown in FIG. 12. The two-piece boom type excavator is modified from the ordinary excavator, shown in FIG. 11, in that a boom 101 of a work front 100A is divided into two parts, i.e., a first boom 104 and a second boom 105. Here, the work front 100A is called a 3-articulation type work front based on the number of articulations which take part in positioning a bucket 103 , and an excavator provided with the work front 100A is called a 3-articulation type excavator.
The 3-articulation type excavator has an advantage of enabling the work to be easily carried out near an undercarriage of the excavator, which has been difficult for the 2-articulation type excavator. More specifically, although the 2-articulation type excavator can also be operated to take a posture shown in FIG. 11 for bringing the bucket 103 to a position near the undercarriage, the excavation work cannot be performed with the arm 102 positioned so horizontally as illustrated. On the other hand, in the 3-articulation type excavator, the bucket 103 can be brought to a position near the undercarriage with the arm 102 held substantially vertical as shown in FIG. 12, allowing the excavation work to be carried out near the undercarriage. Further, the excavation work in a position away from the undercarriage can be performed up to a farther position than reachable with the 2-articulation type excavator by extending the first boom 104 and the second boom 105 so as to lie almost straight.
Another advantage of the 3-articulation type excavator is in enabling the work front to swing with a reduced swing radius. When the direction of the work front 100A is changed by swinging an upper swing structure 106 for loading dug earth and sand on a dump car or the like, it is difficult for the 2-articulation type excavator to reduce the radius necessary for the swing because the boom 101 has a large overall length. In the 3-articulation type excavator, the radius necessary for the swing can be reduced by raising the first boom 104 to take a substantially vertical posture and making the second boom 105 extend substantially horizontally. This means that the 3-articulation type excavator is more advantageous in carrying out the work in a narrow-space site.
Next, the conventional operating method will be explained. FIG. 13 shows one example of control levers for use in an ordinary 2-articulation type excavator. In normal excavation work, four kinds of operations effected by the boom, the arm, the bucket and the swing are carried out frequently in a combined manner. These four kinds of operations are allocated to two control levers 107, 108 such that each control lever instructs the two kinds of operations. The excavation work is performed by an operator manipulating the respective levers with the left and right hands. As another control lever, there is a (not-shown) travel lever (usually associated with a pedal as well). The travel lever is used independently of the other levers 107, 108 in many cases; hence it is not here taken into consideration.
FIG. 14 shows one example of control levers for use in a 3-articulation type excavator. As mentioned above, the 3-articulation type excavator can be operated to carry out the work over a wide range from a further position to a position nearer to its undercarriage. To realize this, however, the second boom 105 must also be operated in addition to the first boom 104 corresponding to the boom 101 of the 2-articulation type excavator. Since the four kinds of operations are already allocated to the two control levers 107, 108, a seesaw type pedal 109 is newly provided to operate the second boom 105.
Further, JP, A, 7-180173 proposes a control system for a 3-articulation type excavator. According to the proposed control system, two control levers are designed to instruct moving speeds of a bucket end in the X- and Y-directions, respectively, and a predetermined calculation process is executed based on a resultant speed vector signal of those moving speeds. As a result, in leveling work, movement of the bucket end can be controlled continuously over a larger area and the bucket can be moved along a desired path with high accuracy.